LDOS 5.3.1 for the Model III - REPAIR/CMD Disassembled

REPAIR/CMD

Other Navigation

LDOS 5.3.1 REPAIR/CMD Disassembly - Making a Diskette of Another DOS Readable (Model III)

REPAIR/CMD changes the system areas of a diskette written by another operating system so that LDOS can read its files. The command is REPAIR :d. The manual (M-40-060) says it is needed on the Model III to read a non-DOS diskette made on the Model I or one made by a Model I LDOS before 5.0.2, that TRSDOS 1.2 and 1.3 diskettes must be copied with CONV instead and never repaired, and that a repaired diskette may no longer be readable by the system that made it, because the data address mark of its directory changes; it advises repairing a backup copy. Its list of what REPAIR does: the directory cylinder gets the LDOS data address mark, DIR/SYS is made readable, the excess cylinder byte is checked, the granules per cylinder byte is set, bit 7 of the directory cylinder byte of the boot sector is stripped (for DOSPLUS diskettes), and the system information sectors are written.

The file is a load module of 517 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, two load blocks for 5200H-53C5H (454 bytes) and the transfer address record for 5200H. Every sector goes through one buffer beyond the file, 5400H-54FFH. REPAIR works with the disk routines of SYS0/SYS: RDSECT (4777H), WRSECT (4763H, normal data address mark), WRPROT (4768H, the deleted data mark the LDOS directory uses) and VERSEC (4772H), with Index Register IY at the drive code table that GETDCT (478FH) finds.

What REPAIR Does

About This Page

Load Blocks in File Order

The two load blocks of the file, after the copyright record: 454 bytes, 5200H-53C5H. The transfer address is 5200H.

Sections of the Disassembly

Memory REPAIR/CMD Uses

The entry points of REPAIR/CMD, then the data of the file, the operand it changes, and the cells outside the file that it uses, by address.

Entry Points

Data in the File

Operands Changed While It Runs

Cells Outside the File That REPAIR/CMD Uses

5200H - Entry: the Banner and the Drive

LDOS Ready loads REPAIR/CMD at 5200H-53C5H and jumps to 5200H, the transfer address, with Register Pair HL at the rest of the command line after the word REPAIR and the spaces after it. The command is REPAIR :d (M-40-060). The Stack Pointer is kept in the operand of LD SP at 52E4H for every exit; the banner is displayed; a colon and a digit 0-7 give the drive, no colon gives drive 0, and a colon with any other character gives error 20H, Illegal drive number. The drive is not checked by @CKDRV: GETDCT only points Index Register IY at its drive code table.

5225H - The Boot Sector: the Directory Cylinder Byte

Cylinder 0, sector 0 is read into the buffer at 5400H. Byte 02H of the boot sector is the directory cylinder (BOOT/SYS of LDOS reads it from there); some other systems keep bit 7 set in it (the manual names DOSPLUS). REPAIR clears bit 7, writes the byte back to the boot sector and into the drive code table (+09H), and writes the sector back with the normal data address mark.

5246H - The GAT: Sides, Cylinders and the Configuration Byte

The GAT (directory cylinder, sector 0) is read into 5400H. Two sides are taken only from a GAT of LDOS: its byte CBH (the version, 53H on this diskette) must be 40H-6FH and bit 5 of byte CDH set. The count of cylinders comes from the lock out table 60H-BFH: the last byte that is not FFH (FFH marks a cylinder the diskette does not have), and byte CCH gets the cylinders less 35. Byte CDH is rebuilt from the drive code table: bit 7 from +04H bit 7 (cylinder 0 in single density), bit 6 from +03H bit 6 (double density), bit 5 from +04H bit 5 (two sides), bits 2-0 from bits 7-5 of +08H (the granules on a track less one); bits 4-3 end clear. The GAT is written back with the deleted data mark and verified.

5293H - The HIT and the Directory Sectors

Sector 1 of the directory cylinder is the hash index table (HIT); its byte 01H belongs to DIR/SYS's directory record (the first record of sector 3) and gets C4H, the hash of the name DIR/SYS. Then sectors 2 up are read and written back with the deleted data mark, the mark of the LDOS directory. In sector 3 the access password of DIR/SYS (+12H-+13H of its record) becomes 4296H, the hash of a blank password, so DIR/SYS can be opened and read; the record's ending record number (+14H, the count of directory sectors) sets how many sectors are done: sectors 2 to that number less 1.

52C2H - Cylinder 0 Again and the End

A loop meant for the sectors of cylinder 0 runs once (Register B = 01H): the boot sector is read again, gets the directory cylinder from +09H of the drive code table, the value 5230H stored, and is written back with the normal data address mark. No system information sector (cylinder 0, sector 2) is written, although the manual lists it. Then Alien disk conversion complete is displayed and REPAIR ends through 52E4H with Register Pair HL as the return code: 0000H for success, FFFFH after an error (then @ABORT during a JCL job).

52F3H - Write With the Deleted Data Mark, Read, and the Error Exits

52F3H writes a directory sector with the deleted data mark and verifies it: the verify must find that mark (error 06H), so it returns the Z FLAG only then. 52FCH reads a sector into 5400H, a read that finds the deleted data mark (06H) counts as good; any other error goes on into 5309H. 5307H (inside the LD BC at 5306H) gives error 20H, 5309H displays the short message of the error in Register A and returns, and both end with Register Pair HL = FFFFH. 5317H (inside the LD IX at 5316H) displays -Nothing done-; 5313H, which would display REPAIR aborted, is not reached by any instruction of the file.

531FH - The Texts, the Drive Cell and the Buffer

The banner, the three messages, then the cell and the buffer beyond the file.